This invention relates to a pipe-type oil-filled (hereinafter referred to as POF) cable line for an electric power transmission system, and more particularly to a POF cable core whose longitudinal thermal expansion can be easily absorbed.
The POF cable generally comprises cable cores 2 and joints 3 connecting the cable cores 2 together in a steel pipe 1 as shown in FIG. 1. The POF cable described above is conventionally installed in a straight line, except for the case making a detour of obstructions lying in the cable-laying route. The longitudinal thermal expansion of the cable core 2 when supplied with electrical load is conventionally absorbed by the snakelike bending 4 of the cable core 2, that is, the cable snaking, which occurs in a relatively broad space between the steel pipe 1 and the cable core 2. (The steel pipe has an inner diameter approximately 2.7 times larger than the outer diameter of the cable core 2.).
Generally the cable snaking 4 of the cable core 2 tends to occur near the cable joint 3 and the thermal expansion of the cable core tends to concentrate at the cable snaking area. Consequently the curvature radius of the cable snaking 4 becomes small and repeated thermal bending will create soft spots and creases in insulation papers, in the case of the paper-insulated cable. And in some extreme cases, such bending will lead to dielectric breakdown.